There has been considerable interest in recent years in the development of a monitoring system for obtaining a continuous measurement of a patient's blood pressure. One of the most promising techniques for obtaining such a continuous measurement involves the use of an arterial tonometer comprising an array of small pressure sensing elements fabricated in a silicon "chip." The use of such an array of sensor elements for blood pressure measurements is disclosed generally in the following U.S. Patents.: U.S. Pat. Nos. 3,123,068 to R. P. Bigliano, 3,219,035 to G. L. Pressman, P. M. Newgard and John J. Eige, 3,880,145 to E. F. Blick, 4,269,193 to Eckerle, and 4,423,738 to P. M. Newgard, and in an article by G. L. Pressman and P. M. Newgard entitled "A Transducer for the Continuous External Measurement of Arterial Blood Pressure" (IEEE Trans. Bio-Med. Elec., April 1963, pp. 77-81).
In a typical tonometric technique for monitoring blood pressure, a transducer which includes an array of pressure sensitive elements is positioned over a superficial artery, and a hold-down force is applied to the transducer so as to flatten the wall of the underlying artery without occluding the artery. The pressure sensitive elements in the array have at least one dimension smaller than the lumen of the underlying artery in which blood pressure is measured, and the transducer is positioned such that more than one of the individual pressure-sensitive elements is over at least a portion of the underlying artery. The output from one of the pressure sensitive elements is selected for monitoring blood pressure. The element that is substantially centered over the artery has a signal output that provides an accurate measure of intraarterial blood pressure. However, for the other transducer elements the signal outputs generally do not provide as accurate a measure of intraarterial blood pressure as the output from the centered element. Generally, the offset upon which systolic and diastolic pressures depend will not be measured accurately using transducer elements that are not centered over the artery. One method for selecting the pressure sensitive element for monitoring blood pressure is disclosed in the above mentioned U.S. Pat. No. 4,269,193 issued to J. S. Eckerle. In addition, an improved method for selecting the correct pressure sensitive element for measuring blood pressure is disclosed in a patent application entitled "Active Element Selection for Continuous Blood Pressure Monitor Transducer" filed on even date herewith.
One of the difficulties encountered in the development of tonometric blood pressure monitoring systems is the correct placement of the transducer on the patient's wrist such that the pressure sensing elements are centered over the underlying artery. The positioning system of the present invention, described in greater detail below, overcomes this difficulty.